1. Field of the Invention
This invention relates to a rolling bearing to be used in the fields of automobiles, construction machines, agricultural machines, electrical equipment, etc., and particularly to a rolling bearing of which the inner race, outer race and/or rolling members have long life, and which can be manufactured with fewer grinding operations and thus at lower cost.
2. Related Background art
In the prior art, for production of a long life bearing, a case hardened steel containing about 0.2% of carbon such as SCr 420H, SAE 4320H, SCM 420H, etc. has been used. Through carburization a hardness at the surface portion as H.sub.R C of 58 to 64 and a hardeness at the core portion as H.sub.R C of 30 to 48 has been obtained.
Generally speaking, a carburized steel bearing has been known to be greater in rolling fatigue life as compared with a noncarburized hardened steel bearing. The reason for this is that when hardening is effected without carburization, the surface portion is cooled and is converted to martensite earlier than the core portion. As a result, strain due to expansion accompanying martensite formation remains in the circumferential direction as a tensile stress in the surface portion and as a compression stress in the core portion. For this reason, generation of so-called quench cracking or surface cracking as observed upon rolling fatigue breaking is promoted. On the other hand, when carburizing is effected, since diffusion of carbon into the austenite structure proceeds from the surface, the solid solution (dissolved) carbon content in the structure becomes more enriched in the surface portion than in the core portion, whereby the Ms point at the surface portion becomes lower as compared with the core portion. As a result, martensitic transformation occurs first at the core portion during hardening, so that there is residual circumferential compression stress in at the surface portion and tensile stress at the core portion. Thus, the generation of quench cracking and surface cracking can be prevented.
However, since the residual stress by carburization is at an equilibrium state between the surface portion and the core portion after hardening, when the surface layer is removed by grinding, the equilibrium is collapsed, thus giving rise to deformation and a problem of out-of-roundness. In order to correct out-of-roundness, the number of grinding operations must be is increased, but this leads to the disadvantage of increased production cost as compared with the noncarburization hardened steel bearing.